1. Field of the Invention
This invention relates to heat treating apparatus and method for heating a substrate, such as mask substrate for photomask, after applying resist liquid on the substrate or after exposing the resist liquid on the substrate and before developing the substrate.
2. Background Art
In the manufacturing process for semiconductor devices, a manufacturing technology called “photo lithography” is employed to form a desired resist pattern on the surface of a semiconductor. Also, for a rectangular mask substrate (reticle substrate) as an exposure mask for a semiconductor device, the lithography is used as well, where resist liquid is firstly applied on the surface of the substrate and secondly, the resulting resist film is exposed using a desired pattern and thereafter, the mask substrate is developed to form a designated resist pattern on the surface of the substrate.
It is noted that the above-mentioned resist liquid is obtained by dissolving components of a coating film in solvent. Therefore, after applying the resist liquid on the substrate, a heat treatment called “baking” where the substrate is heated to a predetermined temperature is performed to volatilize the solvent from the substrate. After the heat treatment, the substrate is cooled down to a designated temperature prior to the exposure. For example, this heat treatment or cool-down treatment is carried out on condition of mounting the substrate on the surface of a mount table equipped with heating unit or cooling unit.
Now, one example of a heating device for heating a substrate is described with reference to FIG. 10, in brief. In the figure, reference numeral 1 denotes a heating plate on which a substrate G is mounted. On the surface of the heating plate 1, a plurality of projections 11 (for example, four projections) are formed to support the back side of the substrate G in a slightly-floated state from the surface of the heating plate 1 through a gap (e.g. interval of approx. 0.5 mm), avoiding an adhesion of particles to the back side of the substrate G. The heating plate 1 is provided, therein, with a heater 12 as the heating unit. Based on a detecting value from a sensing element 13 (e.g. thermocouple), a controller 14 controls an output (i.e. heating operation) of the heater 12 under PID (Proportional Integral Differential) control, so that the substrate G above the plate 1 is heated up to a designated temperature (e.g. 130° C.). Note, reference numeral 15 denotes a lid body for the heating plate 1, which can move up and down. Reference numeral 16 denotes elevating pins for delivering the substrate G onto the heating plate 1.
In the heating device like this, the lid body 15 is elevated to open the device and then, the elevating pins 16 are spaced from the heating plate 1 to receive the substrate G from not-shown transfer mean. On receipt of the substrate G, subsequently, the elevating pins 16 are lowered to mount the substrate G on the heating plate 1. Then, the lid body 15 is also lowered to close up the device. On the formation of a closed processing space above the heating plate 1, a designated heat treatment is applied on the substrate G.
When intending to heat the substrate G, there exists a remarkable temperature difference between the substrate G and the heating plate 1 since the temperature of the substrate G is relatively low, for example, approx. 23° C. Accordingly, when the substrate G is mounted on the heating plate 1, the surface temperature of the heating plate 1 is lowered rapidly due to absorption of heat by the substrate G. If the responsibility of the PID control is established high (i.e. establishment of high setting values for P, I and D) for purpose of recovering the temperature of the heating plate 1 for a designated temperature as possible, then the output of the heater 12 becomes larger, so that the temperature of the heating plate 1 exceeds a target value in overshoot. As a result, with the overshoot of the heating plate 1 in temperature, the temperature of the substrate G may overshoot disadvantageously, as well.
In such a case, there is a possibility of producing great deviations in the thickness of coating films formed on the substrates G. In order to deal with such a disadvantage, if the responsibility of the PID control is established low so as to elevate the temperature of the heating plate 1 moderately in view of avoiding its overshoot in temperature, a long time will be required to recover the temperature of the plate 1 for the target value, also causing a disadvantage. Under the present circumstances, the controller 14 is provided with one control loop for the PID control and another control loop for executing a fixed control (i.e. MV control) to supply the heater 12 with power in accordance with a fixed output pattern while utilizing a timer and a method utilizing the structure. In the method, when the substrate G is mounted on the heating plate 1 as a result of lowering the elevating pins 16, the PID control is switched to the MV control to increase a heat-up rate of the heating plate 1. Then, with the elapse of a predetermined period, the MV control is changed to the PID control (see paragraph No. 0028 to 0030 and FIG. 4 in Japanese Patent Publication No. 11-74187).
However, the technique disclosed in Japanese Patent Publication No. 11-74187 has the following problem. As mentioned above, in order to heat the substrate G, it is necessary to open the lid body 15 of the heating device prior to the operation of not-shown transfer unit to mount the substrate G on the heating plate 1. That is, when opening the lid body 15, a heat radiation is caused through the resulting opening of the heating device to lower the temperature of the heating plate 1. Since the heat radiation occurs the peripheral area of the heating plate 1 at first, the temperature about the peripheral area of the plate 1 comes down in comparison with the temperature of a center area of the plate 1.
Thus, the reduction in the temperature of the peripheral area of the heating plate 1 causes the action of PID control to be accelerated by the time of mounting the substrate G on the plate 1, so that the output of the heater 12 gets larger at the peripheral area. As a result, the temperature of the peripheral area of the heating plate 1 becomes too high in comparison with that of the center area of the plate 1. Then, the substrate G is mounted on the heating plate 1 where its in-plane temperature distribution changes for the worse. Since the MV control does not allow the temperature difference between the peripheral area of the plate 1 and the center area to be redressed, the in-plane temperature distribution of the substrate G in its heat-up period deteriorates consequently.
Thus, it is difficult to accomplish uniform heat treatment in the surface of the substrate G, causing the evaporation rate of solvent in the coating liquid on the substrate G to be inhomogeneous in plane. As a result, there is a worry that the film thickness of the coating film varies in the surface of the substrate. Additionally, since it is difficult to apply uniform heat treatment on the substrates G, the film thickness of the coating film may vary among the substrates G. For the mask substrate, since it is required to have in-plane uniformity with high accuracy, such a problem becomes obvious. However, the above-mentioned publication (No. 11-74187) does not suggest or teach a technique to dissolve such a problem.